The present invention relates to a discharge lamp control device and a projector.
A discharge lamp such as a high-pressure mercury lamp or a metal halide lamp is used as a light source of a projector. When lighting a discharge lamp, a high voltage of several tens of kilovolts is normally generated between electrodes of the discharge lamp in order to cause a dielectric breakdown between the electrodes of the discharge lamp to form a discharge path. The voltage (discharge lamp voltage) between the electrodes of the discharge lamp rapidly decreases immediately after the discharge lamp has been lighted. In order to heat the electrodes of the discharge lamp so that the discharge lamp voltage increases to a value near the rated voltage value, constant current control is performed so that a discharge lamp current which flows between the electrodes of the discharge lamp becomes constant when the discharge lamp voltage is lower than a predetermined voltage value. When the discharge lamp voltage has become equal to or higher than the predetermined voltage value, constant power control is performed so that the amount of power supplied to the discharge lamp becomes constant. According to a related-art method, as shown in FIG. 8A, constant power is supplied to the discharge lamp during constant power control irrespective of the discharge lamp voltage. Since the electrodes of the discharge lamp deteriorate as the period of time in which the discharge lamp is lighted increases, the discharge lamp voltage is increased to supply constant power. Specifically, when the discharge lamp has reached its terminal stage, constant power control is performed in a state in which the discharge lamp voltage is increased to a predetermined voltage value. In this case, the operation of a discharge lamp driver circuit (ballast circuit) is stopped in order to prevent explosion of the discharge lamp so that the discharge lamp stops producing light (i.e., the life of the discharge lamp expires). In a liquid crystal projector device disclosed in JP-A-2000-28988, the amount of power supplied to a discharge lamp is set at a constant value equal to or lower than the rated value from the initial stage of the discharge lamp in order to extend the life of the discharge lamp.
However, since the amount of power supplied to the discharge lamp is set at a constant value equal to or lower than the rated value from the initial stage of the discharge lamp, the luminance decreases from the initial stage of the discharge lamp. According to a related-art method, as shown in FIG. 8B, a discharge lamp current near the maximum design current value of the discharge lamp is caused to flow during constant current control in order to reduce the transition time from constant current control to constant power control. In a discharge lamp lighting device disclosed in JP-A-9-82480, for example, the discharge lamp current during constant current control is set to be higher than the discharge lamp current during constant power control. However, the electrodes of the discharge lamp are melted due to the high discharge lamp current during constant current control, whereby the life of the discharge lamp decreases.
In order to extend the life of the discharge lamp, the discharge lamp may be driven by applying an alternating current so that a current evenly flows between the electrodes of the discharge lamp. According to a related-art method, the drive frequency is set at a constant frequency, as shown in FIG. 8C. However, the electrodes of the discharge lamp are easily damaged due to a large change in current when the polarity of the discharge lamp current changes. Therefore, the electrodes of the discharge lamp deteriorate to a large extent when the drive frequency is too high, whereby the life of the discharge lamp decreases.